Effect of Nd and Nb co-doping on the structural,magnetic and optical properties of multiferroic BiFeO3 nanoparticles prepared by sol-gel method

BiFeO₃, BiFe_0.95Nb_0.05O₃ and Bi_1−xNd_xFe_0.95Nb_0.05O₃ (x=0.0, 0.05, 0.10, 0.15 and 0.20) nanoparticles are successfully synthesized via a tartaric acid-assisted sol-gel technique for the first time. The effect of Nd and Nb co-doping on the structure, morphology, and magnetic and optical properties is investigated. X-ray diffraction (XRD) and Raman measurements demonstrate that Nd³⁺ ions and Nb⁵⁺ ions co-doping at A and B-sites of BiFeO₃ can result in a structural transformation (from single rhombohedral phase (R3c) to two coexisting phases (R3c and Pbam)). The morphology of the nanoparticles seems to be approximately cubic, and by the co-doping, the size of the nanoparticles decreases from ~132 to ~35 nm. Notable improvement in the remanent magnetization of the sample with x=0.15 is observed with a value of 0.285 emu/g, being 20 times higher than that of the undoped sample (BiFeO₃). A decrease in the optical band gap is also observed in the Nd and Nb co-doped nanoparticles, indicating their favorable potential in photocatalytic applications.     

Effect of Dy-substitution on structural,electrical and magnetic properties of multiferroic BiFeO3 ceramics

The polycrystalline samples of dysprosium (Dy)-modified bismuth ferrite (i.e., Bi_1−xDy_xFeO₃; x=0–0.2 with the interval of 0.05) (BDFO) were synthesized using a high-temperature solid-state reaction method. Preliminary X-ray structural analysis showed that the reported crystal structure of BiFeO₃ (rhombohedral) is invariant even with Dy-substitution at the Bi-site upto x=0.2. The scanning electron micrograph of the compounds showed (i) the uniform distribution of grains on the sample surface with high density and (ii) reduction of grain size on increasing Dy content in BiFeO₃ (BFO). Studies of impedance, electrical modulus and electric conductivity of the materials in wide frequency (10–1000 kHz) and temperature (30–500 °C) ranges using a complex impedance spectroscopy technique have provided new and interesting information on the contribution of grains, grain boundary and interface in these parameters. Detailed studies of impedance spectroscopy clearly exhibit the dielectric relaxation of non-Debye type. The ac conductivity of the Dy-substituted BFO obeyed Jonscher's universal power law. An increase in Dy-content in BDFO results in the increase of spontaneous magnetization of BFO due to the collapse of spin cycloid structure.     

Raman and dielectric spectroscopic analysis of magnetic phase transition in Y(Fe0.5Cr0.5)O3 multiferroic ceramics

Here, we report the Raman and dielectric spectroscopic studies as a function of temperature of orthorhombically distorted Y(Fe_0.5Cr_0.5)O₃ (YFC) ceramics, measured from 80 to 300 K. The dc-magnetization measurements under field cooled (FC)-zero field cooled (ZFC) protocol indicate a small onset of magnetic ordering at T_N∼270 K. The field dependent magnetization plot recorded at 50 K, 150 K and 200 K show a clear opening in hysteresis loops. The linear dependence of magnetization plot at high field without any saturation of magnetization indicates the coexistence of weak ferromagnetic (WFM) component within the canting antiferromagnetic (CAFM) matrix. Temperature evolution of Raman line-shape parameter of B_2g(4) phonon mode clearly exhibits an anomalous behavior of phonon shift near T_N∼270 K, indicating the spin-phonon coupling in the ceramics. From the temperature dependent dielectric permittivity (ε(T)) study, two dielectric relaxation peaks are detected below 200 K and above 250 K. The appearance of former relaxation peak is responsible for polaronic conduction mechanism, while the later one is associated with magnetic phase transition which might be relevant to the presence of magnetoelectric coupling in YFC ceramics. The observed P-E hysteresis loops at room temperature indicate weak ferroelectric nature of the ceramics.     

Modulation of magnetic,ferroelectric and leakage properties by HoFeO3 substitution in multiferroic 0.7BiFeO3-0.3Ba0.8Ca0.2TiO3 solid solutions

An investigation on the structural, magnetic, electrical properties of HoFeO₃-substituted 0.7BiFeO₃-0.3Ba_0.8Ca_0.2TiO₃ solid solutions synthesized using conventional solid state reaction method was carried out. The structural study confirms that an additional orthorhombic (Pbnm) phase of HoFeO₃ appears in the ceramic matrix and the presence of the aforementioned phase significantly influences the magnetic characteristics in the solid solutions. A detailed high temperature dielectric study suggests that the oxygen vacancies can be effectively controlled by the appropriate amount of substitution, successively regulating the ferroelectric as well as the leakage properties in the ceramics. Furthermore, the foremost remnant polarization and the feeble dielectric loss is achieved when the substitution content is x~0.2. Therefore, an appropriate amount of HoFeO₃ substitution can be an effective way to modulate the multiferroic properties in the present ceramics.     

Characterization of multiferroic PbFe0.5Nb0.5O3 and PbFe0.5Ta0.5O3 ceramics derived from citrate polymeric precursors

Ceramics of the perovskite multiferroics PbFe_0.5Nb_0.5O₃ (PFN) and PbFe_0.5Ta_0.5O₃ (PFT) were synthesized from new citrate polymeric precursors. X-ray tests pointed to trace amounts of the pyrochlore phase. SEM studies revealed the heterogeneous grain size distribution for PFN and the homogeneous one for PFT. Dielectric studies pointed to one diffuse T-C phase transition at 378 K for PFN and two diffuse M-T and T-C phase transitions, at 200 and 235 K, for PFT, respectively. X-ray photoelectron spectroscopy studies of PFN reveal that all ions exist in one valence state, however, with two chemical shifts for Pb²⁺. Two valence states for the majority of ions of PFT seem to be connected with a higher volume fraction of the amorphous grain-boundary phase. The electronic energy band gap for both compounds is approximately 2.8 eV. Two magnetic transitions, ie, from the paramagnetic to the antiferromagnetic phase and then to the spin-glass phase, were observed at 156 and 10 K for PFN, and at 145 and 15 K for PFT, respectively.     

Effect of calcination temperature on structural,magnetic and optical properties of multiferroic YFeO3 nanopowders synthesized by a low temperature solid-state reaction

Nanosize multiferroic YFeO₃ powders have been synthesized via the low temperature solid-state reaction. Differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy all indicated that the phase-pure orthorhombic YFeO₃ powders were obtained at 800 °C with a size below 150 nm. X-ray photoelectron spectroscopy (XPS) showed the Fe³⁺ ions to be predominant. Magnetic hysteresis loops exhibited some ferromagnetic behaviour of the YFeO₃ nanopowders at ambient temperature. The maximum and remnant magnetizations of the powders were about 2.49 and 0.88 emu/g, respectively. Moreover, optical measurements demonstrated that the optical band gap of the nanopowders was around 2.4 eV, proving that they can strongly absorb visible light. So an easy and efficient way to synthesize YFeO₃ nanopowders with promising application in the magnetic and optical fields has been successfully developed.     

Effect of dysprosium substitution on crystal structure and physical properties of multiferroic BiFeO3 ceramics

The polycrystalline samples of multiferroic Bi_1−xDy_xFeO₃ (x = 0, 0.1, and 0.2) were prepared by a modified solid state reaction method and characterized by X-ray diffraction, scanning electron microscopy, differential thermal analysis, dielectric and magnetic measurements. It was shown that the introduction of the Dy³⁺ ions stabilizes the perovskite structure and improves phase purity. The coexistence of the rhombohedral and orthorhombic phases was found to exist within the investigated concentration range 0.1 ≤ x ≤ 0.2. The changes and anomalies observed in dielectric response over a wide frequency range were correlated with the structural evolution and the development in microstructure. The SQUID measurements of the field-dependent magnetization at different temperatures demonstrated Dy doping to be a very effective method for inducing a weakly ferromagnetic state in the ferroelectric R3c phase of BiFeO₃ in the absence of an external magnetic field.     

Room-temperature magnetic control of ferroelectric polarization and enhanced ferromagnetic properties in 0.8BiFeO3-BaTiO3 ceramic

A multiferroic 0.8BiFeO₃ ??0.2BaTiO₃ (0.8BF-BaT) ceramic was prepared by solid-state reaction to study the magnetic control of its ferroelectric polarization and ferromagnetic properties. The influence of the magnetic field H on the ferroelectric polarization of 0.8BF-BaT ceramic was investigated. The maximum displacement current density Jm and the remanent polarization Pr of 0.8BF-BaT increased by 35% and 20% respectively after it being magnetized, indicating that the polarization was suppressed in the magnetic domain walls. Under a magnetic field of H?=?0.7?T, Jm and Pr decreased by 6.1% and 6.4% respectively, revealing coupling between polarization P and magnetic field H. The magnetic properties of BiFeO₃ were improved by introducing of BaTiO₃ and the magnetic Néel temperature T_N increased by 70?K because of structural effects.     

Enhanced ferroelectric and ferromagnetic properties of BiFeO3-PbTiO3 multiferroic solid solutions with Ba substitutions

Dense 0.6(Bi_0.9La_0.1)(Ga_0.05Fe_0.95)O₃-0.4(Pb_1-xBa_x)TiO₃ (BLGF-PBT, x = 0, 0.1, 0.2, 0.3, and 0.4) ceramics with different Ba contents were prepared by the solid-state reaction method, and effects of Ba contents on the structure and multiferroic properties were investigated. X-ray diffraction results indicate that the Ba-modified BLGF-PBT ceramics exhibit single perovskite structure without detectable secondary phases, and a transition from tetragonal phase to rhombohedral one takes place with the increase of Ba content. The piezoelectric constant d₃₃, remanent polarization P_r, and remanent magnetization M_r are improved by the Ba substitution simultaneously. Values of P_r, M_r, and d₃₃ of BLGF-PBT ceramics for the composition of x = 0.3 with the coexistence of tetragonal and rhombohedral phases are 20 μC/cm², 0.05 emu/g, and 256 pC/N, respectively, much higher than those without Ba modification. The significantly improved d₃₃ and M_r indicate that BLGF-PBT ceramics with Ba modification provide alternative materials for multifunctional devices.     

Correlation between structure,dielectric and multiferroic properties of lead free Ni modified BaTiO3 solid solution

Present study highlights the influence of Ni doping on the structural, optical, dielectric, ferroelectric and magnetic properties of nanocrystalline BaTi_1-xNi_xO₃ (0 ≤ x ≤ 0.06) ceramics synthesized by sol-gel auto combustion process. Phase identification and crystal structure are examined through Rietveld refinement analysis that ensures mono-phase nature with tetragonal crystal structure in P4mm space group. The observed variation in the structural parameters viz. lattice constants, unit cell volume, bond lengths (Ti-O) and bond angles (Ti-O-Ti) are the direct evidence of distortion produce in the unit cell under the effect of Ni doping. The FTIR studies confirm perovskite structure and presence of stretching/bending vibrations of the various bands present in the samples. The optical properties divulge a minor alteration in optical bandgap under the influence of Ni content. Dielectric studies reveal higher value of the dielectric constant for pristine sample in the low frequency region, but its value decreases on Ni doping. The dielectric response, analyzed through UDR model, exhibits deviation from linear behavior at higher frequencies. Ferroelectric measurements demonstrate that the pristine sample has higher values of remnant polarization (P_r) and maximum polarization (P_m) which decrease linearly with the increase in Ni doping. Magnetic hysteresis loops at room temperature establish a weak ferromagnetic nature of the samples that arises due to the carrier-mediated exchange interactions with smaller values of magnetic parameters. These investigations ensure that the ferromagnetism can be induced in BaTiO₃ by appropriate doping of Ni ions, which may find their potential use in the field of multiferroics.     

Structural,electrical, magnetic and magnetoelectric properties of Co-doped BaTiO3 multiferroic ceramics

In this work, BaTi_1-xCo_xO₃ (BTCO) ceramics with x?=?0, 2.5, 5, 7.5 and 10?mol% have been synthesized and their structural, electrical, magnetic and magnetoelectric have been investigated. Rietveld refinement of XRD data reveals that pure BTO has pure tetragonal phase. On the other hand, between 2.5?≤?x?≤?7.5, BTCO shows both tetragonal as well as hexagonal phases. At x?=?10?mol%, BTCO shows only hexagonal phase. The grain size of the BTCO samples is found to increase with Co doping concentration. The ferroelectric polarization and relative permittivity of BTCO samples reduce with an increase in the Co concentration. A standard magnetization equation is used for fitting the magnetic hysteresis (M-H) curve, thus deconvoluting the ferromagnetic (FM) and paramagnetic (PM) components. The saturation magnetization (M_s) gradually increases from x?=?2.5 to x?=?10?mol%, the value being 0.8 memu/g and 8.92 memu/g respectively. The origin of magnetization is due to the oxygen vacancies and their associated exchange interaction. The magnetodielectric coefficient (MD) shows a reducing trend from 1.80 to 0.18 for x?=?2.5 to x?=?10?mol% respectively. The magnetoelectric coefficient $({\alpha }_{\mathit{ME}})$ for x?=?2.5?mol% is 3.399?mV/cm. Oe, while for x?=?10?mol% it is 0.896?mV/cm. Oe.     

Crystal structure transformation and improved dielectric and magnetic properties of La-substituted BiFeO3 multiferroics

Samples of Bi_1−xLa_xFeO₃ with x = 0.1, 0.3, 0.5, and 0.7 have been synthesized by two stage solid state reaction method. Structural characterization was performed using powder x-ray diffraction at room temperature. The crystal structure of perovskite phases are further characterized via Rietveld analysis which revealed a structural transition from R3c symmetry of the parent phase of BiFeO₃ to orthorhombic Pnma symmetry of LaFeO₃. However the intermediate samples with x = 0.3 and 0.5 are bi – phasic (i.e. a combination of rhombohedral R3c and orthorhombic Pbam phases co-exist). Rietveld Refinement presents a good agreement between measured and simulated patterns. The transition from a rhombohedral to orthorhombic unit cell is suggested to be driven by the dilution of the stereochemistry of the lone pair of Bi³⁺ at A- site. M-H hysteresis loops are recorded at room temperature up to a field of 15 kOe. The G-type antiferromagnetic spin structure and the magnetic moment are very sensitive to increasing La concentration at A-site. La substitution transformed antiferromagnetic BiFeO₃ into ferromagnetic which is closely related to the structural phase transition and modification of antiparallel spin structure. Dielectric constant (ε′) and dissipation factor (tan δ) measured in frequency range 1 kHz to 5 MHz showed dispersion behaviour at low frequencies.     

Effect of Cr doping on ferroelectric and magnetic properties of Bi0.8Ba0.2FeO3

Multiferroic ceramics Bi_0.8Ba_0.2Fe_1−xCr_xO₃ (x=0, 0.05 and 0.1) were synthesized by using the conventional solid state reaction method. The pure phase with rhombohedral structure was confirmed by the X-ray diffraction measurements for all samples. It was shown that the magnetic and the ferroelectric properties were simultaneously improved, and the maximum values of the remnant magnetization (2Mr) and the remnant polarization (2Pr) at room temperature for all samples are around 1 emu/g and 1.9 μC/cm², respectively. Furthermore, the leakage current density, the low frequency dispersion in the dielectric constant and the dielectric loss decreased with increasing the Cr concentration from x=0 to 0.1. A remarkable change in the P–E loop was observed whether a bias dc magnetic field was applied or not, approving the existence of the magnetoelectric coupling indirectly therein.     

Dielectric and magnetic properties of (Pb,Ti) co-doped BiFeO3 multiferroic ceramics

In this study, Bi_0.9Pb_0.1Fe_1-xTi_xO₃ (0.05 ≤ x ≤ 0.20) multiferroic ceramics were prepared through solid-state reaction. The influence of Pb, Ti partial substitutions on the dielectric and magnetic properties of BiFeO₃ multiferroic ceramics was investigated and discussed in detail. X-ray diffractions confirm rhombohedral perovskite phase formation (R3c space group). Scanning electron microscopy (SEM) was employed to investigate the morphology, revealing a cuboidal microstructure with bimodal distribution of grain sizes. Magnetic studies were carried out and the results reveal a slight enhancement of saturation magnetization with Ti concentration increasing. The present data indicates that Bi_0.9Pb_0.1Fe_1-xTi_xO₃ can be used as multifunctional material in different magnetoelectric applications.     

Structural,dielectric, vibrational and magnetic properties of Sm doped BiFeO3 multiferroic ceramics prepared by a rapid liquid phase sintering method

Rare earth Sm substituted Bi_1−xSm_xFeO₃ with x=0, 0.025, 0.05, 0.075 and 0.10 polycrystalline ceramics were synthesized by a rapid liquid phase sintering method. The effect of varying composition of Sm substitution on the structural, dielectric, vibrational, optical and magnetic properties of doped BiFeO₃ (BFO) ceramics have been investigated. X-ray diffraction patterns of the synthesized rare earth substituted multiferroic ceramics showed the pure phase formation with distorted rhombohedral structure with space group R3c. Good agreement between the observed and calculated diffraction patterns of Sm doped BFO ceramics in Rietveld refinement analysis of the X-ray diffraction patterns and Raman spectroscopy also confirmed the distorted rhombohedral perovskite structure with R3c symmetry. Dielectric measurements showed improved dielectric properties and magnetoelectric coupling around Néel temperature in all the doped samples. FTIR analysis establishes O–Fe–O and Fe–O stretching vibrations in BiFeO₃ and Sm-doped BiFeO₃. Photoluminescence (PL) spectra showed visible range emissions in modified BiFeO₃ ceramics. The magnetic hysteresis measurements at room temperature and 5 K showed the increase in the magnetization with the increase in doping concentration of Sm which is due to the structural distortion and partial destruction of spin cycloid caused by Sm doping in BFO ceramics.     

The microstructure,ferroelectric and dielectric behaviors of BiFeO3-Na0.5Bi0.5TiO3 based solid solution thin films

BiFe_0.98Zn_0.02O₃-Na_0.5Bi_0.5Ti_0.98W_0.02O₃ solid solution thin films with two thicknesses (300 nm and 1.2 μm) were fabricated on indium tin oxide/glass substrates via metal organic decomposition. The effects of the thickness on crystallization, microstructure morphology, ferroelectric and dielectric properties were investigated. Compared with the 300-nm-thick film, 1.2 μm- film exhibits standard ferroelectric hysteresis loop with slim feature and larger dielectric constant due to the improvements of crystallinity and insulating property. Moreover, obvious aging behavior, manifested by pinched-like ferroelectric hysteresis loop and abnormal butterfly dielectric constant-electric field curve, can be observed in the film with 1.2 μm thickness. The aging behavior can be explained by the formation of the defect complex.     

Crystal structure refinement,enhanced magnetic and dielectric properties of Na0.5Bi0.5TiO3 modified Bi0.8Ba0.2FeO3 ceramics

(1-x) Na_0.5Bi_0.5TiO₃–x(Bi_0.8Ba_0.2FeO₃) (x=0.5, 0.6, 0.7, and 0.8) ceramics were synthesized via solid state reaction method. Powder X-ray diffraction investigations performed at room temperature along with Rietveld analysis show all the composites to exhibit a rhombohedral distorted perovskite structure, described by space group R3c. Rietveld refinement confirmed a good agreement between observed and calculated intensities and a low value of goodness of fit (χ²). Magnetic measurements were carried out at room temperature up to a field of 6 kOe. Magnetic properties of BBFO modified NBT ceramics are improved with a significant opening in the M–H hysteresis loop at room temperature. Remanent magnetization and coercive field increased with increase of BBFO concentration. The dielectric response of these samples was analyzed in the frequency range 10 Hz–7 MHz at different temperatures revealing a dispersion in dielectric constant (ε′) and in dissipation factor (tan δ) at lower frequencies. Both ε′ and tan δ increase with increase of BBFO content. The temperature dependence of frequency exponent ′s′ of power law suggests that quantum mechanical tunneling (QMT) model to be applicable at lower temperature and correlated barrier hopping (CBH) mechanism to be appropriate at higher temperature to describe the conduction mechanism in x=0.5 and x=0.6 samples. Further, with increase in BBFO content, the dielectric constant becomes more stable at higher frequencies and temperatures thereby improving the dielectric properties of the material.     

Influence of Eu and Sr co-substitution on multiferroic properties of BiFeO3

Pure BiFeO₃ (BFO), and Eu-Sr co-substituted BFO samples were prepared by a sol–gel method. The effects of Eu and Sr codoped on the structural, morphological, magnetic and ferroelectric properties were systematically investigated. The X-ray diffraction and Fourier transform infrared spectroscopy reveal that substitution of Eu and Sr at the Bi site results in structural change and single phase formation. The maximum remnant magnetization of 0.287 emu/g and coercive field of 10.305 kOe are observed in the Bi_0.85Eu_0.05Sr_0.10FeO₃ sample. The suppression of spin cycloid caused from the structural distortion can play an important role in the improvement of magnetic properties. The Eu and Sr co-doped samples also exhibit good ferroelectric properties, which may be attributed to suppressing the formation of oxygen vacancies by Eu substitution.     

Influence of Y doping on structural,vibrational, optical and magnetic properties of BiFeO3 ceramics prepared by Mechanical Activation

Y substituted BiFeO₃ (Bi_1−xY_xFeO_3; x=0.0–0.1) polycrystalline ceramics were synthesized by Mechanical Activation. The effect of varying composition of Y substitution on the structural, vibrational, optical and magnetic properties of doped BiFeO₃ (BFO) ceramics has been investigated. Rietveld refinement of X-ray diffraction patterns reveals that all samples crystallize in distorted rhombohedral structure with R3c symmetry and no structural transition has been observed. Raman spectroscopy also confirmed the distorted perovskite structure with R3c space group. Optical studies in the spectral range 1–4.5 eV were dominated by two d-d and three charge transfer (C-T) transitions. The optical band gap decreases from 2.11 to 2.01 eV with increasing Y substitution. Room temperature magnetic measurements showed weak ferromagnetic ordering and enhancement in magnetization with increasing Y concentration. Mechanical activation leads to significantly altered magnetic properties, particularly in higher Y-doping samples. The Mössbauer spectra demonstrate the suppression of spiral spin modulation of the magnetic moments resulting in enhanced ferromagnetism with increasing doping concentration. Significant increase in Néel temperature T_N in the substituted compounds was discussed on the basis of structural distortions.     

Synthesis,electrical and magnetic characteristics of Nd-modified BiFeO3

The polycrystalline Nd-modified bismuth ferrite BiFeO₃ (Bi_1−xNd_xFeO₃ (BNFO) (x=0, 0.05, 0.15, and 0.25)) were prepared in a single-phase using a standard and cost effective solid-state reaction method. In order to check the quality and formation of the compounds x-rays diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive x-ray spectroscopy (EDAX) techniques were used. Preliminary structural analysis indicates that the crystal structure of BNFO is rhombohedra for its low content of Nd (x=0, 0.05, 0.15) whereas for higher content (x=0.25) it is tetragonal. The dielectric and ferroelectric properties of BiFeO₃ (BFO) were dramatically enhanced on the above Nd-substitutions. Study of the frequency dependence of ac conductivity suggests that the materials obey Jonscher׳s universal power law. An increase in Nd-content in BNFO results in the enhancement of spontaneous magnetization of BFO because of the collapse of spin cycloid structure.